Increased space-time yield in gas phase polymerization

ABSTRACT

The space time yield of a gas phase reactor, particularly a polyethylene reactor may be increased by replacing at least 80 weight % of the ballast gas with a gas having a higher heat capacity than the ballast gas. Preferably the gas replacing the ballast gas is a stream of dilute ethylene having a high concentration of ethane.

FIELD OF THE INVENTION

The present invention relates to the polymerization of one or moreolefins in a gas phase polymerization process having a low per passconversion. Typically in gas phase polymerization of olefins the feedstream passes through a fluidized or stirred bed of growing polymerparticles. The monomer in the feed stream contacts the catalyst in thegrowing polymer particles and is polymerized. The unreacted monomer,ballast gas, typically nitrogen, and molecular weight control agent(typically hydrogen) optionally together with a condensable gas arerecycled through a compressor and heat exchanger to cool the recyclestream and optionally condense the condensable gases. The recycle streamis then made up with additional feed stream and returned to the bed ofgrowing polymer. The polymerization of olefins is exothermic. In gasphase polymerization the removal of heat from the bed of growing polymertends to be a rate limiting step, for a given reactor configuration. Thepresent invention seeks to address this issue by replacing at least 80%of the ballast gas in the feed stream with a gas having a higher heatcapacity than the ballast gas. The present invention also providesintegrating with crackers allowing use of dilute ethylene in a gas phaseprocess.

BACKGROUND OF THE INVENTION

U.S. Pat. Nos. 4,543,399 and 4,588,790 issued to Jenkins, III. et al.Sep. 24, 1985 and May 13, 1986, respectively, assigned to Union CarbideCorporation teach incorporating into the feed stream up to about 20weight % of the recycle stream of a condensable gas. That is a gas whichcondenses when a compressed recycle stream passes through a heatexchanger prior to being recycled back to the reactor. Typically thesegases are C₄₋₆ alkanes, preferably isomers of pentane and hexane.Interestingly, Jenkins does not suggest replacing any portion of theballast gas with either or both of a condensable gas or a gas having ahigher heat capacity.

U.S. Pat. Nos. 5,462,999 and 5,436,304 to Griffin et al. issued Oct. 31,1995 and Jul. 25, 1995, respectively, and U.S. Pat. Nos. 5,405,922 and5,352,749 to DeChellis et al. issued Apr. 11, 1995 and Oct. 4, 1994,respectively, all assigned to Exxon Chemical Patents, Inc. all teachoperating a gas phase polymerization where in the feed stream maycontain from about 17.5 up to 50 weight % of a condensable gas. However,the specification still teaches the feed stream comprises “inerts”preferably nitrogen. No other “inerts” are suggested or disclosed by theabove patents. The patents do not suggest that the process could befurther enhanced by replacing nitrogen with a gas (which may optionallybe condensable) having a higher heat capacity.

U.S. Pat. No. 5,981,818 issued Nov. 9, 1999 to Purvis et al., assignedto Stone & Webster Engineering Corp. teaches the use of dilute ethylenein a number of processes. The dilute ethylene feed may comprise from 1up to 50 weight % of ethane. One process disclosed is the gas phasepolymerization of ethylene. The disclosure cautions that for gas phaseprocesses the dilute ethylene should comprise at least about 95 weight %of ethylene (Col. 9 lines 20-25). This is a higher ethylene content inthe feed gas than in accordance with the present invention. That is thefeed gas in accordance with the present invention comprises less than 95weight % of ethylene and the dilute ethylene in accordance with thepresent invention comprises less than 95 weight % of ethylene.

U.S. Pat. No. 6,111,156 issued Aug. 29, 2000 to Oballa et al. disclosesthe use of dilute ethylene in an integrated polymerization process whichhas a high per pass conversion. The high per pass conversion must begreater than 85%. The per pass conversion for conventional gas phasepolymerization is typically substantially less than 85% (e.g. 2-10%conversion per pass).

The present invention seeks to provide a process to increase theefficiency of a gas phase polymerization having a low per passconversion without significantly raising the dew point of the feedstream (i.e. the condensable phase is not greater than about 25 weight%, preferably less than about 20 %, most preferably less than about 17weight %).

The present invention also seeks to provide a means of integrating a gasphase reactor with an olefin cracker.

SUMMARY OF THE INVENTION

The present invention provides a process to increase the space timeyield of a low per pass conversion gas phase polymerization of one ormore C₂₋₆ alpha olefins comprising replacing not less than 80% of theballast gas in the feed stream to a gas phase reactor with a gas havinga heat capacity greater than the heat capacity of the ballast gas anddoes not significantly raise the dew point of the feed stream.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram of a gas phase polymerizationprocess.

DETAILED DESCRIPTION OF THE INVENTION

The existing process of the gas phase polymerization of ethylene will bedescribed with reference to FIG. 1.

In the gas phase polymerization of polyethylene a gaseous feed stream 1comprising ethylene and one or more C₃₋₆ copolymerizable monomerstypically butene or hexene or both, together with a ballast gas such asnitrogen, optionally a small amount of C₁₋₂ alkanes (i.e. methane andethane) and further optionally a molecular weight control agent(typically hydrogen) is fed to a reactor 2. Typically the feed streampasses through a distributor plate 3 at the bottom of the reactor andtraverses a polymer bed 4, typically a fluidized polymer bed. A smallproportion of the olefin monomers in the feed stream react with thecatalyst. The unreacted monomer and the other non-polymerizablecomponents in the feed stream exit the bed and typically enter adisengagement zone 5 where the velocity of the feed stream is reduced sothat entrained polymer falls back into the fluidized bed. Typically thegaseous stream leaving the top of the reactor is then passed through acompressor 7, via recycle line 6. The compressed gas is then cooledthrough heat exchanger 8 to remove the heat of reaction. The heatexchanger may be operated at temperatures below about 65° C., preferablyat temperatures from 20° C. to 50° C.

Polymer is removed from the reactor through a series of vessels 9. Thepolymer is recovered through line 10 and further processed. The offgases (consisting of monomers and inerts) are fed to a monomer recoveryunit 11 via line 12. The monomer recovery unit may be selected fromthose known in the art including a distillation tower (i.e. a C₂splitter), a pressure swing adsorption unit and a membrane separationdevice. Additionally, a portion of the stream from the recycle line 6may be fed to the monomer recovery unit 11 via line 13 to balance thereactor compositions. Monomer/lnerts product from the monomer recoveryunit is fed via line 14 to a treatment device, typically a flare stacknot shown or partially recycled back to the reactor/discharge system.Heavier components such as C₃₋₅ condensable gases etc. are fed back tothe reactor via line 15 and recycle line 6. Ethylene and hydrogen gasrecovered from the monomer recovery unit are fed back to the reactor vialine 16 and recycle line 6. Ethane recovered from the monomer recoveryunit is fed via line 17 to a further treatment unit, typically a crackershown at 18.

Finally, make up feed stream is added via line 1 and recycle line 6 tothe reactor 2 below distributor plate 3.

The process is described for example in U.S. Pat. No. 4,543,399 issuedSep. 24, 1985 to Jenkins, III et al. In the Jenkins patent the feedstream may contain up to about 20 weight % of a condensable gas at theconditions in the heat exchanger. That is the temperature, (partial)pressure and concentration of the condensable gas exceed the dew pointfor the condensable gas and a portion of it condenses to a liquid whichis entrained in the recycle gas which is fed back to the reactor (belowa distributor plate) and the feed stream with entrained liquids passesthrough the fluidized bed. As the feed stream passes through thefluidized bed the entrained liquids evaporate and remove some of theheat of reaction from the fluidized bed of catalyst. Some condensablegases include C₄₋₆ alkanes (e.g. butane, pentane, iso-pentane, hexane,cyclohexane, etc.)

If there is no condensable gas, the heat of reaction is removed byheating the gas as it passes through the fluidized bed and cooling itwhen it passes through the heat exchanger.

The per pass con version of monomer in the feed stream is low, typicallyless than 10%, generally less than 5%, usually less than about 3%. Thereactors a re operated at moderate temperatures typically less than 120°C., generally from about 80° C. to about 115° C., preferably from 80° C.to 110° C. The operating pressures of a gas phase polymerization systemmay be from about 75 psi (pounds per square inch) to 1200 psi, typicallyfrom 100 psi to 1000 psi , preferably from about 100 to 350 psi.

Several types of catalysts may be used in the gas phase polymerizationprocess. The catalyst may be a Ziegler Natta type catalyst. Typicallythese catalysts comprise a transition metal compound, an aluminum andoptionally a zinc compound, a magnesium compound, and optionally anelectron donor (Lewis base) on a support.

The Ziegler Natta catalysts typically have the formulaM((O_(a))R¹)_(b)X_(c) wherein M is a transition metal preferably Ti; ais 0 or 1; b and c may be 0 or an integer or contain fractions (i.e.1.5) and the sum of b and c is the valence of the transition metal 3 or4; R¹ is selected from the group consisting of C₁₋₈, preferably a C₁₋₄alkyl radical; and X is a halogen atom preferably chloride. One usefultitanium compound is TiCl₄.

The aluminum compound is typically an aluminum alkyl or alkoxide complexor an aluminum alkyl halide. These compounds may be characterized by theformula Al ((O)_(a)R¹)_(d)X_(e) wherein a, R¹, and X are as definedabove and the sum of d and e is 3. While it may be preferred that d ande are integers they may be or contain fractions (i.e. 1.5). Suchcompounds include triethyl aluminum (TEAL), trimethyl aluminum (TMA),diethyl aluminum chloride (DEAC), tri-n-hexyl aluminum (TNHAL), aluminumsesquichloride and mixtures thereof.

The zinc compounds if present is typically a zinc halide such as zincchloride (ZnCl₂).

The magnesium compound may be a magnesium halide (MgCl₂), a magnesiumalkyl compound (R¹MgX wherein R¹ and X are as defined above) or adialkyl magnesium compound (i.e. (R¹)₂Mg where R¹ is as defined above).As the magnesium halides tend to be insoluble in organic solvents theyare typically dissolved as a dialkyl magnesium compound (i.e. diethylmagnesium or butyl ethyl magnesium) and then reacted with halide(typically an organic halide such as R¹X wherein R¹ and X are as definedabove) to produce a fine suspension of MgCl₂.

Optionally the Ziegler Natta catalyst may contain an electron donor(Lewis base). Typically the electron donors are ethers which typicallymay contain up to about 8 carbon atoms (e.g. R¹—O—R¹). The ethers may beacyclic composed of two aliphatic radicals joined through an oxygen atom(diethyl ether) or they may be cyclic ethers such as tetrahydrofuran.Tetrahydrofuran is a commercially available electron donor. If presentthe electron donor may be present in amounts to provide a molar ratio ofelectron donor to transition metal up to about 50:1, preferably lessthan about 25:1, most preferably from about 5:1 to about 15:1.

The catalysts are supported. The supports useful in accordance with thepresent invention typically comprise a substrate of aluminum or silicahaving a pendant reactive moiety. The reactive moiety may be a siloxyradical or more typically is a hydroxyl radical. The preferred supportis silica or alumina. The support should have a particle size from about10 to 250 microns, preferably from about 30 to 150 microns. The supportshould have a large surface area typically greater than about 3 m²/g,preferably greater than about 50 m²/g, most preferably from 100 m²/g to1,000 m²/g. The support will be porous and will have a pore volume fromabout 0.3 to 5.0 ml/g, typically from 0.5 to 3.0 ml/g. Supports, whichare specifically designed to be an agglomeration of subparticles arealso useful.

It is important that the support be dried prior to the initial reactionwith the catalyst or a catalyst component such as an aluminum compound.Generally the support may be heated at a temperature of at least 200° C.for up to 24 hours, typically at a temperature from 500° C. to 800° C.for times from about 2 to 20 hours. The resulting support will be freeof adsorbed water and should have a surface hydroxyl content from about0.1 to 5 mmol/g of support , preferably from 0.5 to 3 mmol/g.

Optionally the aluminum compound may be added as a “split addition”.That is a portion of the aluminum is added to the support and a portionis added to the catalyst at a later stage. A first aluminum compound(Al¹) may be deposited upon the support by contacting the support,preferably silica with an anhydrous solution of an aluminum compound asdefined above wherein a is 0. Most preferably, R¹ is selected from thegroup consisting of methyl, ethyl and butyl radicals. Preferably, c is0. From a commercial viewpoint, an available compound istriethylaluminum (TEAL).

The support is reacted with an aluminum compound such that the amount ofaluminum on the support is from about 0.1 to about 3 weight %,preferably from about 0.5 to about 2 weight %, based on the weight ofthe silica.

The molar ratios of Mg:Al:Ti may vary over wide ranges depending on thebalance of properties required in the catalyst and in the resultingpolymer.(e.g. molecular weight, melt index, felt flow ratio etc.). U.S.Pat. No. 4,302,566 issued Nov. 24, 1981 to Karol et al. and U.S. Pat.No. 4,302,565 issued Nov. 24, 1981 to Goeke et al., both assigned toUnion Carbide Corporation, teach a molar ratio of Mg:Ti:Al of 0.5 to 56,preferably 1 to 10:1:10 to 400, preferably 10 to 100. Other referenceswhich teach catalysts or their process of manufacture include U.S. Pat.No. 6,140,264 issued Oct. 31, 2000 to Kelly et al; U.S. Pat. No.5,633,419 issued May 27, 1997 to Spencer and European Patent 0 595 571granted Jan. 2, 1997 assigned to BP (the texts of which are hereinincorporated by reference).

The catalyst may be activated with a co-catalyst, typically an aluminumalkyl compound as described above wherein a is 0. The catalyst is fed toa fluidized bed gas phase reactor. U.S. Pat. No. 4,543,399 issued Sep.24, 1985 to Jenkins, III. et al. (assigned to Union Carbide Corporation)teaches it is particularly preferred to continuously feed catalyst tothe reactor using the device of U.S. Pat. No. 3,779,712 (issued Dec. 18,1973 to Calvert et al. also assigned to Union Carbide Corporation, nowexpired).

The catalyst may be a chrome base catalyst, typically CrO₃ on a supportas described above.

The catalyst may be a single site type catalyst typically comprising atransition metal, preferably an early transition metal (e.g. Ti, V, Zrand Hf) and generally having two bulky ligands. In many of the wellknown single site catalysts typically one of the bulky ligands is acyclopentadienyl-type ligand. These cyclopentadienyl-type ligandscomprise a C₅₋₁₃ ligand containing a 5-membered carbon ring havingdelocalized bonding within the ring and bound to the metal atom throughcovalent η⁵ bonds which are unsubstituted or may be further substituted(sometimes referred to in a short form as Cp ligands).Cyclopentadienyl-type ligands include unsubstituted cyclopentadienyl,substituted cyclopentadienyl, unsubstituted indenyl, substitutedindenyl, unsubstituted fluorenyl and substituted fluorenyl. An exemplarylist of substituents for a cyclopentadienyl-type ligand includes thegroup consisting of C₁₋₁₀ hydrocarbyl radicals (including phenyl andbenzyl radicals), which hydrocarbyl substituents are unsubstituted orfurther substituted by one or more substituents selected from the groupconsisting of a halogen atom, preferably a chlorine or fluorine atom anda C₁₋₄ alkyl radical; a C₁₋₈ alkoxy radical; a C₆₋₁₀ aryl or aryloxyradical; an amido radical which is unsubstituted or substituted by up totwo C₁₋₈ alkyl radicals; a phosphido radical which is unsubstituted orsubstituted by up to two C₁₋₈ alkyl radicals; silyl radicals of theformula —Si—(R)₃ wherein each R is independently selected from the groupconsisting of hydrogen, a C₁₋₈ alkyl or alkoxy radical, and C₆₋₁₀ arylor aryloxy radicals; and germanyl radicals of the formula Ge—(R)₃wherein R is as defined directly above.

If there are two such bulky ligands (i.e. bis Cp) the catalysts aremetallocene-type catalysts. The Cp ligand may be bridged to another Cpligand by a silyl bridge or a short chain (C₁₋₄) alkyl radical. TheCp-type ligand may be bridged to an amido radical which may be furthersubstituted by up to two additional substituents. Such bridged complexesare sometimes referred to as constrained geometry catalysts.

Broadly, the transition metal complex (or catalyst) suitable for use inthe present invention has the formula:

(L)_(n)—M—(X)_(p)

wherein M is a transition metal preferably selected from Ti, Hf and Zr(as described below); L is a monanionic ligand selected from the groupconsisting of a cyclopentadienyl-type ligand, a bulky heteroatom ligand(as described below) and a phosphinimine ligand (as described below); Xis an activatable ligand which is most preferably a simple monanionicligand such as alkyl or a halide (as described below); n may be from 1to 3, preferably 2 or 3; and p may be from 1 to 3, preferably 1 or 2,provided that the sum of n+p equals the valence state of M, and furtherprovided that two L ligands may be bridged by a silyl radical or a C₁₋₄alkyl radical.

If one or more of the L ligands is a phosphinimine ligand the transitionmetal complex may be of the formula:

wherein M is a transition metal preferably selected from Ti, Hf and Zr(as described below); Pl is a phosphinimine ligand (as described below);L is a monanionic ligand selected from the group consisting of acyclopentadienyl-type ligand or a bulky heteroatom ligand (as describedbelow); X is an activatable ligand which is most preferably a simplemonanionic ligand such as an alkyl or a halide (as described below); mis 1 or 2; n is 0 or 1; and p is an integer fixed by the valence of themetal M (i.e. the sum of m+n+p equals the valence state of M).

In one embodiment the catalysts are group 4 metal complexes in thehighest oxidation state. For example, the catalyst may be a bis(phosphinimine) dichloride complex of titanium, zirconium or hafnium.Alternately, the catalyst contains one phosphinimine ligand, one “L”ligand (which is most preferably a cyclopentadienyl-type ligand) and two“X” ligands (which are preferably both chloride).

The preferred metals (M) are from Group 4, (especially titanium, hafniumor zirconium) with titanium being most preferred.

The catalyst may contain one or two phosphinimine ligands which arecovalently bonded to the metal. The phosphinimine ligand is defined bythe formula:

wherein each R³ is independently selected from the group consisting of ahydrogen atom; a halogen atom; C₁₋₂₀, preferably C₁₋₁₀ hydrocarbylradicals which are unsubstituted by or further substituted by a halogenatom; a C₁₋₈ alkoxy radical; a C₆₋₁₀ aryl or aryloxy radical; an amidoradical; a silyl radical of the formula:

—Si—(R²)₃

wherein each R² is independently selected from the group consisting ofhydrogen, a C₁₋₈ alkyl or alkoxy radical, and C₆₋₁₀ aryl or aryloxyradicals; and a germanyl radical of the formula:

Ge—(R²)₃

wherein R² is as defined above.

The preferred phosphinimines are those in which each R³ is a hydrocarbylradical, preferably a C₁₋₆ hydrocarbyl radical. A particularly preferredphosphinimine is tri-(tertiary butyl) phosphinimine (i.e. wherein eachR³ is a tertiary butyl group).

Preferred phosphinimine catalysts are Group 4 organometallic complexeswhich contain one phosphinimine ligand (as described above) and oneligand L which is either a cyclopentadienyl-type ligand or aheteroligand.

As used herein, the term “heteroligand” refers to a ligand whichcontains at least one heteroatom selected from the group consisting ofboron, nitrogen, oxygen, phosphorus or sulfur. The heteroligand may besigma or pi-bonded to the metal. Exemplary heteroligands includeketimide ligands, silicone-containing heteroligands, amido ligands,alkoxy ligands, boron hetrocyclic ligands and phosphole ligands, all asdescribed below.

As used herein, the term “ketimide ligand” refers to a ligand which:

(a) is bonded to the transition metal via a metal-nitrogen atom bond;

(b) has a single substituent on the nitrogen atom (where this singlesubstituent is a carbon atom which is doubly bonded to the N atom); and

(c) has two substituents Sub 1 and Sub 2 (described below) which arebonded to the carbon atom.

The substituents “Sub 1” and “Sub 2” may be the same or different.Exemplary substituents include hydrocarbyls having from 1 to 20 carbonatoms, silyl groups, amido groups and phosphido groups. For reasons ofcost and convenience it is preferred that these substituents both behydrocarbyls, especially simple alkyls and most preferably tertiarybutyl.

Silicon containing heteroligands are defined by the formula:

—(μ)SiR_(x)R_(y)R_(z)

wherein the—denotes a bond to the transition metal and μ is sulfur oroxygen.

The substituents on the Si atom, namely R_(x), R_(y) and R_(z) arerequired in order to satisfy the bonding orbital of the Si atom. The useof any particular substituent R_(x), R_(y) or R_(z) is not especiallyimportant to the success of this invention. It is preferred that each ofR_(x), R_(y) and R_(z) is a C₁₋₂ hydrocarbyl group (i.e. methyl orethyl) simply because such materials are readily synthesized fromcommercially available materials.

The term “amido” is meant to convey its broad, conventional meaning.Thus, these ligands are characterized by (a) a metal-nitrogen bond and(b) the presence of two substituents (which are typically simple alkylor silyl groups) on the nitrogen atom.

The terms “alkoxy” and “aryloxy” is also intended to convey itsconventional meaning. Thus, these ligands are characterized by (a) ametal oxygen bond and (b) the presence of a hydrocarbyl group bonded tothe oxygen atom. The hydrocarbyl group may be a C₁₋₁₀ straight chained,branched or cyclic alkyl radical or a C₆₋₁₃ aromatic radical whichradicals are unsubstituted or further substituted by one or more C₁₋₄alkyl radicals (e.g. 2,6 di-tertiary butyl phenoxy).

Boron heterocyclic ligands are characterized by the presence of a boronatom in a closed ring ligand. This definition includes heterocyclicligands which also contain a nitrogen atom in the ring. These ligandsare well known to those skilled in the art of olefin polymerization andare fully described in the literature (see, for example, U.S. Pat. Nos.5,637,659; 5,554,775 and the references cited therein).

The term “phosphole” is also meant to convey its conventional meaning.“Phospholes” are cyclic dienyl structures having four carbon atoms andone phosphorus atom in the closed ring. The simplest phosphole is C₄PH₄(which is analogous to cyclopentadiene with one carbon in the ring beingreplaced by phosphorus). The phosphole ligands may be substituted with,for example, C₁₋₂₀ hydrocarbyl radicals (which may, optionally, containhalogen substituents); phosphido radicals; amido radicals; or silyl oralkoxy radicals. Phosphole ligands are also well known to those skilledin the art of olefin polymerization and are described as such in U.S.Pat. No. 5,434,116 (Sone, to Tosoh).

The term “activatable ligand” or “leaving ligand” refers to a ligandwhich may be activated by the alumoxane (also referred to as an“activator”) to facilitate olefin polymerization. Exemplary activatableligands are independently selected from the group consisting of ahydrogen atom; a halogen atom, preferably a chlorine or fluorine atom; aC₁₋₁₀ hydrocarbyl radical, preferably a C₁₋₄ alkyl radical; a C₁₋₁₀alkoxy radical, preferably a C₁₋₄ alkoxy radical; and a C₅₋₁₀ aryl oxideradical; each of which said hydrocarbyl, alkoxy, and aryl oxide radicalsmay be unsubstituted by or further substituted by one or moresubstituents selected from the group consisting of a halogen atom,preferably a chlorine or fluorine atom; a C₁₋₈ alkyl radical, preferablya C₁₋₄ alkyl radical; a C₁₋₈ alkoxy radical, preferably a C₁₋₄ alkoxyradical; a C₆₋₁₀ aryl or aryloxy radical; an amido radical which isunsubstituted or substituted by up to two C₁₋₈, preferably C₁₋₄ alkylradicals; and a phosphido radical which is unsubstituted or substitutedby up to two C₁₋₈, preferably C₁₋₄ alkyl radicals.

The number of activatable ligands depends upon the valence of the metaland the valence of the activatable ligand. The preferred catalyst metalsare Group 4 metals in their highest oxidation state (i.e. 4⁺) and thepreferred activatable ligands are monoanionic (such as ahalide—especially chloride, or C₁₋₄ alkyl—especially methyl). One usefulgroup of catalysts contain a phosphinimine ligand, a cyclopentadienylligand and two chloride (or methyl) ligands bonded to the Group 4 metal.In some instances, the metal of the catalyst component may not be in thehighest oxidation state. For example, a titanium (III) component wouldcontain only one activatable ligand.

As noted above, one group of catalysts is a Group 4 organometalliccomplex in its highest oxidation state having a phosphinimine ligand, acyclopentadienyl-type ligand and two activatable ligands. Theserequirements may be concisely described using the following formula forthe preferred catalyst:

wherein: M is a metal selected from Ti, Hf and Zr; Pl is as definedabove, but preferably a phosphinimine wherein R³ is a C₁₋₆ alkylradical, most preferably a t-butyl radical; L is a ligand selected fromthe group consisting of cyclopentadienyl, indenyl and fluorenyl ligandswhich are unsubstituted or substituted by one or more substituentsselected from the group consisting of a halogen atom, preferablychlorine or fluorine; C₁₋₄ alkyl radicals; and benzyl and phenylradicals which are unsubstituted or substituted by one or more halogenatoms, preferably fluorine; X is selected from the group consisting of achlorine atom and C₁₋₄ alkyl radicals; m is 1; n is 1; and p is 2.

In one embodiment of the present invention the transition metal complexmay have the formula: [(CP)_(q)M[N=P(R³)]_(f)X_(g) wherein M is thetransition metal; Cp is a C₅₋₁₃ ligand containing a 5-membered carbonring having delocalized bonding within the ring and bound to the metalatom through covalent η⁵ bonds and said ligand being unsubstituted or upto fully substituted with one or more substituents selected from thegroup consisting of a halogen atom, preferably chlorine or fluorine;C₁₋₄ alkyl radicals; and benzyl and phenyl radicals which areunsubstituted or substituted by one or more halogen atoms, preferablyflurorine; R³ is a substituent selected from the group consisting ofC₁₋₁₀ straight chained or branched alkyl radicals, C₆₋₁₀ aryl andaryloxy radicals which are unsubstituted or may be substituted by up tothree C₁₋₄ alkyl radicals, and silyl radicals of the formula —Si—(R)₃wherein R is C₁₋₄ alkyl radical or a phenyl radical; L is selected fromthe group consisting of a leaving ligand; q is 1 or 2; f is 1 or 2; andthe valence of the transition metal−(q+f)=g.

The activator may be selected from the group consisting of:

(i) an aluminoxane; and

(ii) an activator capable of ionizing the transition (Group 4) metalcomplex (which may be used in combination with an alkylating activator).

The single site catalysts may be activated using alumoxanes. Alumoxaneshave the formula (R⁴)₂AlO(R⁴AlO)_(m)Al(R⁴)₂ wherein each R⁴ isindependently selected from the group consisting of C₁₋₂₀ hydrocarbylradicals, m is from 3 to 50. Preferably m is from 5 to 30. Mostpreferably R⁴ is selected from the group consisting of C₁₋₆, mostpreferably C₁₋₄ straight chained or branched alkyl radicals. Suitablealkyl radicals include a methyl radical, an ethyl radical, an isopropylradical and an isobutyl radical. In some commercially availablealumoxanes R⁴ is a methyl radical.

The catalyst useful in accordance with the present invention may have amolar ratio of aluminum from the alumoxane to transition metal from 5 to300: 1, preferably from 25 to 200:1, most preferably from 50 to 120:1.Typically the alumoxane loading on the support will be from 1 to 40weight % based on the (weight of the) support, preferably from 2 to 30weight % based on the (weight of the) support, most preferably from 5 to20 weight % based on the (weight of the) support. The correspondingloading of transition metal from the single site catalyst will be withinthe above specified ratio of Al:transition metal. Generally the loadingof transition metal on the support will be from 0.01 to 5 weight % basedon the (weight of the) support, preferably from 0.05 to 2 weight % oftransition metal based on the (weight of the) support, most preferablyfrom 0.1 to 1 weight % of transition metal based on the (weight of the)support.

An activator capable of ionizing the transition metal complex may beselected from the group consisting of:

(i) compounds of the formula [R⁵]⁺[B(R⁷)₄]⁻ wherein B is a boron atom,R⁵ is a cyclic C₅₋₇ aromatic cation or a triphenyl methyl cation andeach R⁷ is independently selected from the group consisting of phenylradicals which are unsubstituted or substituted with from 3 to 5substituents selected from the group consisting of a fluorine atom, aC₁₋₄ alkyl or alkoxy radical which is unsubstituted or substituted by afluorine atom; and a silyl radical of the formula —Si—(R⁹)₃; whereineach R⁹ is independently selected from the group consisting of ahydrogen atom and a C₁₋₄ alkyl radical; and

(ii) compounds of the formula [(R⁸)_(t)ZH]⁺[B(R⁷)₄ ]⁻ wherein B is aboron atom, H is a hydrogen atom, Z is a nitrogen atom or phosphorusatom, t is 2 or 3 and R⁸ is selected from the group consisting of C₁₋₈alkyl radicals, a phenyl radical which is unsubstituted or substitutedby up to three C₁₋₄ alkyl radicals, or one R⁸ taken together with thenitrogen atom may form an anilinium radical and R⁷ is as defined above;and

iii) compounds of the formula B(R⁷)₃ wherein R⁷ is as defined above.

In the above compounds preferably R⁷ is a pentafluorophenyl radical, andR⁵ is a triphenylmethyl cation, Z is a nitrogen atom and R⁸ is a C₁₋₄alkyl radical or R⁸ taken together with the nitrogen atom forms ananilinium radical which is substituted by two C₁₋₄ alkyl radicals.

The activator capable of ionizing the transition metal complex abstractone or more L¹ ligands so as to ionize the transition metal center intoa cation but not to covalently bond with the transition metal and toprovide sufficient distance between the ionized transition metal and theionizing activator to permit a polymerizable olefin to enter theresulting active site. In short the activator capable of ionizing thetransition metal complex maintains the transition metal in a +1 valencestate, while being sufficiently liable to permit its displacement by anolefin monomer during polymerization.

Examples of compounds capable of ionizing the transition metal complexinclude the following compounds:

triethylammonium tetra(phenyl)boron,

tripropylammonium tetra(phenyl)boron,

tri(n-butyl)ammonium tetra(phenyl)boron,

trimethylammonium tetra(p-tolyl)boron,

trimethylammonium tetra(o-tolyl)boron,

tributylammonium tetra(pentafluorophenyl)boron,

tripropylammonium tetra(o,p-dimethylphenyl)boron,

tributylammonium tetra(m,m-dimethylphenyl)boron,

tributylammonium tetra(p-trifluoromethylphenyl)boron,

tributylammonium tetra(pentafluorophenyl)boron,

tri(n-butyl)ammonium tetra(o-tolyl)boron,

N,N-dimethylanilinium tetra(phenyl)boron,

N,N-diethylanilinium tetra(phenyl)boron,

N,N-diethylanilinium tetra(phenyl)n-butylboron,

N,N-2,4,6-pentamethylanilinium tetra(phenyl)boron,

di-(isopropyl)ammonium tetra(pentafluorophenyl)boron,

dicyclohexylammonium tetra(phenyl)boron

triphenylphosphonium tetra(phenyl)boron,

tri(methylphenyl)phosphonium tetra(phenyl)boron,

tri(dimethylphenyl)phosphonium tetra(phenyl)boron,

tropillium tetrakispentafluorophenyl borate,

triphenylmethylium tetrakispentafluorophenyl borate,

benzene (diazonium) tetrakispentafluorophenyl borate,

tropillium phenyltris-pentafluorophenyl borate,

triphenylmethylium phenyl-trispentafluorophenyl borate,

benzene (diazonium) phenyltrispentafluorophenyl borate,

tropillium tetrakis (2,3,5,6-tetrafluorophenyl) borate,

triphenylmethylium tetrakis (2,3,5,6-tetrafluorophenyl) borate,

benzene (diazonium) tetrakis (3,4,5-trifluorophenyl) borate,

tropillium tetrakis (3,4,5-trifluorophenyl) borate,

benzene (diazonium) tetrakis (3,4,5-trifluorophenyl) borate,

tropillinum tetrakis (1,2,2-trifluoroethenyl) borate,

triphenylmethylium tetrakis (1,2,2-trifluoroethenyl) borate,

benzene (diazonium) tetrakis (1,2,2-trifluoroethenyl) borate,

tropillium tetrakis (2,3,4,5-tetrafluorophenyl) borate,

triphenylmethylium tetrakis (2,3,4,5-tetrafluorophenyl) borate, and

benzene (diazonium) tetrakis (2,3,4,5-tetrafluorophenyl) borate.

Readily commercially available activators which are capable of ionizingthe transition metal complexes include:

N,N- dimethylaniliniumtetrakispentafluorophenyl borate;

triphenylmethylium tetrakispentafluorophenyl borate; and

trispentafluorophenyl boron.

If the transition (e.g. Group 4) metal complex is activated with acombination of an aluminum alkyl compound other than aluminoxane and acompound capable of ionizing the transition metal complex the molarratios of transition metal:metal in the aluminum alkyl compound;metalloid (i.e. boron or phosphorus) in the activator capable ofionizing the transition metal complex (e.g. boron) may range from 1:0:1to 1:10:5.

In accordance with the present invention at least a significant portion,preferably not less than 80%, most preferably not less than 95%,desirably not less than 98% of the ballast gas in the feed stream isreplaced with a gas having a heat capacity greater than that of theballast gas. In one embodiment of the invention the gas replacing theballast gas contains less than 5%, preferably less than 2.5% and mostpreferably no gas which is condensable to a liquid when the compressedfeed stream passes through the heat exchanger.

In one embodiment of the invention the gas replacing the ballast gas inthe feed stream is a source of dilute or unpurified ethylene. The streamreplacing the ballast stream may be a stream from an ethylene crackerwhich has only been treated to remove the methane and the hydrocarbonshaving more than 6 carbon atoms. Typically such a stream comprises 75%to 95% of ethylene and from 25 to 2.2 weight % of ethane and from 0 to2.5 weight % of one or more non-polymerizable C₃₋₅ gases condensable attemperatures from 20° C. to 50° C. at the pressures of the heatexchanger for the recycle stream.

The gas replacing the ballast gas may comprise from 10 to 50% of thefeed stream. As a result the feed stream may comprise, in addition tothe gas replacing the ballast gas from 0 to 20 weight % of one or moreC₄₋₆ alpha olefins; from 0 to 25 weight % of one or more additionalnon-polymerizable C₃₋₅ gases condensable at temperatures from 20° C. to50° C. at the pressures of the heat exchanger for the recycle stream;and from 0 to 1.0 weight % of hydrogen. The resulting feed stream mayalso comprise up to 0.5 weight % of nitrogen. Compared to the feedstream having ballast gas, the feed stream in accordance with thepresent invention may comprise up to an additional 42.5 weight % ofethane and up to about an additional 50 weight % of ethylene.

Overall the feed stream of the present invention comprises from 30 to75, preferably from 30 to 50 weight % of ethylene, from 20 to 50,preferably from 24 to 50 weight % of ethane, from 0 to 20 weight % ofone or more C₄₋₆ copolymerizable alpha olefins, from 0 to 25 weight % ofone or more C₃₋₅ gases condensable at temperatures from 20° C. to 50° C.at the pressures of the heat exchanger for the recycle stream and from 0to 1 weight % of hydrogen the sum of the components being selected toadd up to 100 weight % of the feed stream.

Preferably the feed stream of the present invention comprises from 35 to45 weight % of ethylene, from 50 to 24 weight % of ethane, from 10 to 20weight % of one or more copolymerizable alpha olefins selected from thegroup consisting of butene and hexene, from 10 to 20 weight % of one ormore C₃₋₅ gases condensable at temperatures from 20° C. to 50° C. at thepressures of the heat exchanger for the recycle stream and from 0 to 1weight % of hydrogen.

The process of the present invention may be operated in “condensingmode”. Accordingly, the process may further comprise recovering recyclefeed stream from the reactor and from the monomer recovery loop andcompressing the feed stream and passing the resulting compressed feedstream through a heat exchanger to condense that portion of the feedstream condensable at temperatures (e.g. from 20° C. to 50° C.) and thepressures of the heat exchanger for the recycle stream. One may thenrecycle to the gas phase polymerization reactor that portion of thecondensed recycle stream to keep the amount of liquids entrained in thegas phase constant within process control limits.

In accordance with the present invention the ethane may be recoveredfrom the recycle stream, preferably after it is compressed. The ethanemay be recovered using a number of technologies such as a C₂ splitter (adistillation tower capable of separating ethane from ethylene) apressure swing adsorption unit, or a membrane separation unit. At leasta part, and preferably all of the separated and/or recovered ethyleneand/or hydrogen from the recycle stream is fed back to the gas phasepolymerization reactor. Preferably at least a part, most preferably notless than 85%, desirably not less than 95 weight % of the ethanerecovered from the feed stream is recycled to an ethylene cracker.

The present invention is particularly suitable for use in a turnkeychemical complex where there is a cracker (either ethane or oil ornaphtha) and a gas phase polyethylene plant. Accordingly it is notnecessary to significantly purify the ethylene prior to polymerizationand the recovered ethane from the recycle stream can be recycleddirectly back to the cracker.

The present invention will now be illustrated by the following example.

EXAMPLE 1

Example 1 is a computer generated analysis showing the mass balance in aprocess according to the present invention and the prior art inaccordance with FIG. 1 in which the monomer recovery unit is C₂splitter. The computer model used to generate the data is used andcorrectly predicts the mass balance in a commercially operated gas phasepolyethylene plant of NOVA Chemicals Corporation.

In the computer model the feed stream comprised 26,746 kg of ethyleneper hour; 1,511 kg/hour of comonomer; 0.973 kg/hr of hydrogen and 1,542kg/hr of nitrogen. The vent off the recycle stream was 153 kg per hour.A stream of purge gas to the polymer discharge system was 1,324 kg/hour.The monomer recovery unit had an output of 1,944 kg/hr of gas which canbe returned to the reactor or vented to flare. Liquids at a rate of1,775 kg/hour are sent back to the reactor. The output of polymer was29,550 kg/hr.

The computer simulation was then re-run using a condensed mode (e.g. inaccordance with U.S. Pat. No. 4,543,399 to Jenkins, III et al. andreplacing all of the ballast gas (N₂) with ethane and also running indry mode (no condensable gas) and replacing the ballast gas with ethane.

The results are set forth in Table 1 below.

TABLE 1 Dry Mode Rate Improvements Condensed Mode Rate ImprovementsSimulation With Simulation With Simulation With Simulation With EthaneNitrogen in Ethane in Nitrogen Replacing Nitrogen Dry Mode Dry Mode(Prior Art) (Invention) (Prior Art) (Invention) Molar CompositionEthylene 0.3346 0.3340 0.3338 0.3337 1-Hexene 0.03838 0.03831 0.038300.03828 Hydrogen 0.04601 0.04593 0.04591 0.04589 Nitrogen 0.4795 0.00000.5120 0 Condensable Gas 0.03175 0.04838 0 0 Ethane 0.06968 0.53330.06996 0.5821 Weight % Liquids 7.00 7.00 0.000 0.000 Production Rate29550 35727 14252 21552

The results from Table 1 show that in condensing mode of operation theincrease in productivity when replacing all of the nitrogen withessentially ethane is about 20%. More impressive is the result in thedry mode of operation (i.e. no condensable gas) which shows an increasein productivity of about 50%.

What is claimed is:
 1. A process to increase the space time yield of agas phase polymerization of one or more C₂₋₆ alpha olefins having a perpass conversion of less than 5% of alpha olefin to polyolefin with gaspassing through a reactor and being cycled through a heat exchanger andback to the reactor being operated at a temperature of from 80° C. to115° C. and a pressure from 100 psi to 1000 psi in the presence of acatalyst selected from the group consisting of Ziegler Natta catalystsand transition metal complexes containing at least one C₅₋₁₃ ligandselected from the group consisting of a cyclopentadienyl radical, anindenyl radical and a fluorenyl radical which are unsubstituted orfurther substituted by up to the number of available carbon atoms withsubstituents selected from the group consisting of a halogen atom, andC₁₋₄ alkyl radicals, and mixtures thereof said transition metal complexhaving been activated with an aluminoxane or an activator capable ofionizing the transition metal complex in which process not less than 80weight % of the ballast gas in the feed stream to a gas phase reactor isreplaced with a gas comprising from 75 weight % to 95 weight % ofethylene; from 25 to 2.2 weight % of ethane and from 0 to 2.5 weight %of one or more non-polymerizable C₃₋₅ gases condensable at temperaturesfrom 20° C. to 50° C. at the pressures of the heat exchanger for therecycle stream.
 2. The process according to claim 1, wherein the gasreplacing the ballast gas comprises from 10 to 50 weight % of the gas inthe feed stream.
 3. The process according to claim 2, wherein the feedstream further comprises from 0 to 20 weight % of one or more C₄₋₆ alphaolefins; from 0 to 25 weight % of one or more additionalnon-polymerizable C₃₋₅ gases condensable at temperatures from 20° to 65°C. at pressures of the heat exchanger for the recycle stream; and from 0to 1.0 weight % of hydrogen.
 4. The process according to claim 3,wherein the feed stream further comprises up to 0.5 weight % ofnitrogen.
 5. The process according to claim 4, wherein the feed streamfurthers comprises up to 42.5 weight % of additional ethane.
 6. Theprocess according to claim 5, wherein the feed stream further comprisesup to 50 weight % of additional ethylene.
 7. The process according toclaim 6, wherein the stream replacing the ballast stream is a streamfrom an ethylene cracker which has only been treated to remove themethane and hydrocarbons having more than 6 carbon atoms.
 8. The processaccording to claim 7, further comprising passing at least part of therecycle stream from the reactor through a heat exchanger to condensethat portion of the recycle stream condensable at temperatures from 20°C. to 65° C. at the pressures of the heat exchanger.
 9. The processaccording to claim 8, further comprising recycling to the gas phasepolymerization reactor a portion of the condensed recycle stream to keepthe amount of liquids entrained in the gas phase constant within processcontrol limits.
 10. The process according to claim 9, further comprisingpassing a portion of the uncondensed recycle stream through a pressureswing adsorption unit to remove ethane from the stream.
 11. The processaccording to claim 10, further comprising recycling at least a portionof the ethane removed from the uncondensed recycle stream to an ethylenecracker.
 12. The process according to claim 11, further comprisingrecycling at least a part of the ethylene, hydrogen or both recoveredfrom the uncondensed recycle stream to the gas phase reactor.
 13. Theprocess according to claim 9, further comprising passing a portion ofthe uncondensed recycle stream through a C₂ splitter to remove ethanefrom the stream.
 14. The process according to claim 13, furthercomprising recycling at least a portion of the ethane removed from theuncondensed recycle stream to an ethylene cracker.
 15. The processaccording to claim 14, further comprising recycling at least a part ofthe ethylene, hydrogen or both recovered from the uncondensed recyclestream to the gas phase reactor.
 16. The process according to claim 9,further comprising passing a portion of the uncondensed recycle streamthrough a membrane separation unit to remove ethane from the stream. 17.The process according to claim 13, further comprising recycling theethane removed from the compressed uncondensed recycle stream to anethylene cracker.
 18. The process according to claim 14, furthercomprising recycling at least a part of the ethylene, hydrogen or bothrecovered from the compressed uncondensed recycle stream to the gasphase reactor.
 19. The process according to claim 12, wherein thecatalyst is a Ziegler Natta catalyst.
 20. The process according to claim15, wherein the catalyst is a Ziegler Natta catalyst.
 21. The processaccording to claim 18, wherein the catalyst is a Ziegler Natta catalyst.22. The process according to claim 12, wherein the catalyst is atransition metal complex containing at least one C₅₋₁₃ ligand selectedfrom the group consisting of a cyclopentadienyl radical, an indenylradical and a fluorenyl radical which are unsubstituted or furthersubstituted by up to the number of available carbon atoms withsubstitutents selected from the group consisting of a halogen atom, andC₁₋₄ alkyl radicals, and mixtures thereof.
 23. The process according toclaim 15, wherein the catalyst is a transition metal complex containingat least one C₅₋₁₃ ligand selected from the group consisting of acyclopentadienyl radical, an indenyl radical and a fluorenyl radicalwhich are unsubstituted or further substituted by up to the number ofavailable carbon atoms with substitutents selected from the groupconsisting of a halogen atom, and C₁₋₄ alkyl radicals, and mixturesthereof.
 24. The process according to claim 18, wherein the catalyst isa transition metal complex containing at least one C₅₋₁₃ ligand selectedfrom the group consisting of a cyclopentadienyl radical, an indenylradical and a fluorenyl radical which are unsubstituted or furthersubstituted by up to the number of available carbon atoms withsubstitutents selected from the group consisting of a halogen atom, andC₁₋₄ alkyl radicals, and mixtures thereof.
 25. The process according toclaim 22, wherein said activator is an aluminoxane of the formula R⁴₂AlO(R⁴AlO)_(m)AlR⁴ ₂ wherein each R⁴ is independently selected from thegroup consisting of C₁₋₄ hydrocarbyl radicals and m is from 5 to 30 andis present in an amount to provide a molar ratio of aluminum totransition metal from 50:1 to 1000:1.
 26. The process according to claim23, wherein said activator is an aluminoxane of the formula R⁴₂AlO(R⁴AlO)_(m)AlR⁴ ₂ wherein each R⁴ is independently selected from thegroup consisting of C₁₋₄ hydrocarbyl radicals and m is from 5 to 30 andis present in an amount to provide a molar ratio of aluminum totransition metal from 50:1 to 1000:1.
 27. The process according to claim24, wherein said activator is an aluminoxane of the formula R⁴₂AlO(R⁴AlO)_(m)AlR⁴ ₂ wherein each R⁴ is independently selected from thegroup consisting of C₁₋₄ hydrocarbyl radicals and m is from 5 to 30 andis present in an amount to provide a molar ratio of aluminum totransition metal from 50:1 to 1000:1.
 28. The process according to claim22, wherein said activator is capable of ionizing the transition metalcomplex selected from the group consisting of: (i) compounds of theformula [R⁵]⁺[B(R⁷)₄]⁻ wherein R⁵ is a cyclic C₅₋₇ aromatic cation or atriphenyl methyl cation and each R⁷ is independently selected from thegroup consisting of phenyl radicals which are unsubstituted orsubstituted with from 3 to 5 substituents selected from the groupconsisting of a fluorine atom, a C₁₋₄ alkyl or alkoxy radical which isunsubstituted or substituted by a fluorine atom; and a silyl radical ofthe formula —Si—(R⁹)₃; wherein each R⁹ is independently selected fromthe group consisting of a hydrogen atom and a C₁₋₄ alkyl radical; (ii)compounds of the formula [(R⁸)_(t)ZH]⁺[B(R⁷)₄]⁻ wherein H is a hydrogenatom, Z is a nitrogen atom or phosphorus atom, t is 2 or 3 and Z isnitrogen or phosphorus and R⁸ is selected from the group consisting ofC₁₋₈ alkyl radicals, a phenyl radical which is unsubstituted orsubstituted by up to three C₁₋₄ alkyl radicals, or one R⁸ taken togetherwith the nitrogen atom may form an anilinium radical and R⁷ is asdefined above; and (iii) compounds of the formula B(R⁷)₃ wherein R⁷ isas defined above.
 29. The process according to claim 23, wherein saidactivator is capable of ionizing the transition metal complex selectedfrom the group consisting of: (i) compounds of the formula[R⁵]⁺[B(R⁷)₄]⁻ wherein R⁵ is a cyclic C₅₋₇ aromatic cation or atriphenyl methyl cation and each R⁷ is independently selected from thegroup consisting of phenyl radicals which are unsubstituted orsubstituted with from 3 to 5 substituents selected from the groupconsisting of a fluorine atom, a C₁₋₄ alkyl or alkoxy radical which isunsubstituted or substituted by a fluorine atom; and a silyl radical ofthe formula —Si—(R⁹)₃; wherein each R⁹ is independently selected fromthe group consisting of a hydrogen atom and a C₁₋₄ alkyl radical; (ii)compounds of the formula [(R⁸)_(t)ZH]⁺[B(R⁷)₄]⁻ wherein H is a hydrogenatom, Z is a nitrogen atom or phosphorus atom, t is 2 or 3 is Z isnitrogen or phosphorus and R⁸ is selected from the group consisting ofC₁₋₈ alkyl radicals, a phenyl radical which is unsubstituted orsubstituted by up to three C₁₋₄ alkyl radicals, or one R⁸ taken togetherwith the nitrogen atom may form an anilinium radical and R⁷ is asdefined above; and (iii) compounds of the formula B(R⁷)₃ wherein R⁷ isas defined above.
 30. The process according to claim 24, wherein saidactivator is capable of ionizing the transition metal complex selectedfrom the group consisting of: (i) compounds of the formula[R⁵]⁺[B(R⁷)₄]⁻ wherein R⁵ is a cyclic C₅₋₇ aromatic cation or atriphenyl methyl cation and each R⁷ is independently selected from thegroup consisting of phenyl radicals which are unsubstituted orsubstituted with from 3 to 5 substituents selected from the groupconsisting of a fluorine atom, a C₁₋₄ alkyl or alkoxy radical which isunsubstituted or substituted by a fluorine atom; and a silyl radical ofthe formula —Si—(R⁹)₃; wherein each R⁹ is independently selected fromthe group consisting of a hydrogen atom and a C₁₋₄ alkyl radical; (ii)compounds of the formula [(R⁸)_(t)ZH]⁺[B(R⁷)₄]⁻ wherein H is a hydrogenatom, Z is a nitrogen atom or phosphorus atom, t is 2 or 3 and Z isnitrogen or phosphorus and R⁸ is selected from the group consisting ofC₁₋₈ alkyl radicals, a phenyl radical which is unsubstituted orsubstituted by up to three C₁₋₄ alkyl radicals, or one R⁸ taken togetherwith the nitrogen atom may form an anilinium radical and R⁷ is asdefined above; and (iii) compounds of the formula B(R⁷)₃ wherein R⁷ isas defined above.